Door sets are great for external doors, either uPVC or wooden, never used them internally however.

Not sure how well building block work around a door set would be: all the ones that I have come across have been fitted into existing block work or new stud work. Presumably you mean that you'll just get the bricky to leave an approximately correct sized gap and then you'll wedge in the door set at a later day? Positioning the door set first and building around it seems like you're asking for the door set to be out of true which can mean the door catches or rubs in the frame, loosing all the benefit of getting pre-hung in the first place.

The door sets that I've come across have a three-sided frame, the long sides of which extend beyond the door and need to be cut off to the right length. The short side is the top. Remember this. It's important. Especially if you've got rising hinges.

I recently project managed a university building with 60 odd door sets, all of which we bought pre-assembled. The doors sets were bespoke; you need to be accurate measuring the opening if installing into a pre-existing opening, or build accurately to the drawings.

Pre-hung double doors can be heavy; think through access and manual handling issues before buying; we had a couple of door locations where we had to strip down the door sets to fit them due to weight and location issues.

Also, if you have complex locking and hinge arrangements (we automated actuators using sensors in the hinges), you need to ensure that the installers respect the doors. Similarly, make sure tradesmen are careful with the finishes.

Otherwise they are brilliant; you get factory quality standards rather than site.